Waters have been monitored spectrophotometrically for many years. However, generally only one chemical or physical parameter was measured at a time, and measurements were generally made using a single optical technique, such as light scattering for turbidity or fluorescence for chlorophyll-a. Typically, water monitoring occurs sporadically by occasional field sampling, which is taken to a laboratory for analysis. Some specialized sensors have been used on water flowing through a pipe, which provide real-time measurement of one component of the water. Generally it is desirable to move laboratory instruments to the field to perform continuous on-line water monitoring but such monitoring has been for a very limited number of water components. Typically, measurement of multiple components using more than one optical technique has required multiple sensors. This approach requires a skilled operator capable of performing multiple calibration and maintenance procedures. Multiple sensors also propagate errors from each device, which limits the monitoring system's ability to accurately detect anomalies or events using a combination of readings.
Several relationships have been established in the art. The Beer-Lambert Law relates to absorbance readings. This is most often used in a quantitative way to determine concentrations of an absorbing species in solution:A=log10(I0/I)=ε1c     where A is the measured absorbance, I0 is the incident light intensity at a given wavelength, I is the transmitted light intensity, 1 the path length through the sample, and c the concentration of the absorbing species. For each species and wavelength, ε is a constant known as the molar absorptivity or extinction coefficient. This constant is a fundamental molecular property in a given solvent, at a particular temperature and pressure, and has units of 1/M*cm or often AU/M*cm.
The Beer-Lambert Law predicts a linear relationship between absorption and concentration and is useful for characterizing many compounds but does not hold as a universal relationship. A second order polynomial relationship between absorption and concentration is sometimes encountered for very large, complex molecules or simpler compounds at relatively high concentration. The Beer-Lambert law has implicit assumptions that must be met experimentally for it to apply. For instance, the chemical makeup and physical environment of the sample can alter its extinction coefficient. The chemical and physical conditions of a test sample therefore must match reference measurements for conclusions to be valid. The Beer-Lambert law also only applies to pure solutions and unencumbered absorbance. In the real world, scattering from particles and non-specific absorption contribute to measured values. The ability to measure the sample at multiple optical path lengths allows the molecular absorptivity to be calculated directly for the sample being evaluated at the wavelength being used.
One apparatus for measuring the purity of fluids is known from the disclosure of U.S. Pat. No. 8,102,518 to Haught et al. Devices used for measuring fluid purity in general, and for identifying and quantifying the amount of impurities in particular, commonly use light as a probing mechanism. Such devices are generally referred to as photometers. A specific type of photometer is the spectrophotometer, which permits adjustment of the light frequency (i.e., Wavelength), for making measurements at multiple frequencies. An optical sample cell contains a portion of fluid being analyzed at any given moment.
Electromagnetic energy that is used to irradiate the aqueous stream may either be rejected by material in the aqueous stream, transmitted through the aqueous stream and its material load, or absorbed by the aqueous stream material. In the instance where the electromagnetic energy is absorbed by the aqueous stream material, the aqueous stream material may also fluoresce. In devices used to measure purity, one of three basic measurement methodologies following from these potential interactions of the electromagnetic energy with the aqueous stream is generally employed. These methodologies measure the parameters absorption, reflectance, and fluorescence of the aqueous stream in the optical sample cell. In accomplishing the various methodologies, an electromagnetic energy detector has been disposed with respect to an electromagnetic energy transmitter so that the detector is optimally positioned to be responsive to the associated parameter.